Seamless and untrimmed primary servo burst with multiple secondary servo bursts

ABSTRACT

A disk storage system having servo data stored thereon for accurate head position control over a magnetic or other storage medium. The servo data is seamless and untrimmed and consists of a primary servo burst set (AB burst) in which the difference defines the desired tract pitch for the surface. Multiple secondary servo burst sets (e.g., CD burst and EF burst) are written at the desired track pitch but radially offset from the primary burst set. In one embodiment, two secondary servo burst sets are used, but alternatively, any number of secondary servo burst sets could be used. In this exemplary embodiment having two secondary servo burst sets, the CD servo burst set is offset by ⅓ of the track pitch from the primary servo burst set. Also in this embodiment, the EF servo burst set is offset by ⅔ of the track pitch from the primary servo burst set. There are six bursts per set. The servo data is written in multiple servo sectors around the surface of the medium. From the servo data, three PES (position error signals) are generated, A−B, C−D and E−F, for providing linear position sensitivity information to the head position control logic. The resulting PES is more linear for smaller head sizes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from co-pending U.S. patent applicationSer. No. 10/104,649, filed on Mar. 21, 2002, entitled “SEAMLESS ANDUNTRIMMED PRIMARY SERVO BURST WITH MULTIPLE SECONDARY SERVO BURSTS,” byFukushima, and assigned to the assignee of the present application,which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of data storage devices. Morespecifically, embodiments of the present invention relate to an improvedservo burst pattern for a hard disk drive.

2. Related Art

A disk storage system, such as a magnetic hard disk drive (HDD), usesone or more disks or “platters” as a data recording medium. The HDDrecords data on the disk by use of a head which can also reproduce datafrom the disk.

Increased levels of storage capacity in floppy and hard disk drives area direct result of the higher track densities possible with voice-coiland other types of servo positioners used today. Previously, low trackdensity disk drives were able to achieve satisfactory head positioningwith lead screw and stepper motor mechanisms. However, when trackdensities are so great that the mechanical error of a lead screw-steppermotor combination is significant compared to track-to-track spacing; anembedded servo-pattern became necessary so that the position of the headcan be determined from the signals it reads off of the storage medium.

As a result, conventional hard disk manufacturing techniques ofteninclude writing servo-patterns (or servo “bursts”) on the media of ahead disk assembly (HDA) with a specialized servowriter instrument.Conventional disk drive assemblies typically include one or more disks,which include a plurality of concentric tracks that are radiallydisplaced from each other on the surface of the disk for storing data.During disk fabrication, servo data is written on the disk by aservowriting process to delineate the centerlines of the tracks. Duringsubsequent disk operations, the servo data is also read by a read/writehead to provide information regarding the position of the head withrespect to the track. The head position information enables a servocontroller to re-align the head over a track when position errors aredetected. Conventional servo-patterns typically comprise short bursts,very precisely located offset from a track's center line, on eitherside. Each track has a preset number of data sectors arranged betweenthe servo sectors and user data is recorded in the data sector. Theservo information may include cylinder data (track address code) usedfor the seeking operation and servo burst data.

The servo bursts are written in a sector header area, and can be used tofind the center line of a track. Staying on track center is requiredduring both reading and writing. These servo-pattern areas allow a headto follow a track center line around a disk, even when the track is outof round, as can occur with spindle wobble, disk slip and/or thermalexpansion.

More specifically, the bursts are generally, but not required to be,located in a trajectory within a track. The servo burst data isconstructed by a plurality of burst patterns for deriving a positionalerror (position data) of the head in a range of the target head or in arange to the adjacent track after the head is moved to a position nearthe target track by the seeking operation. The processor of the headpositioning control circuit converts the amplitude of a position signalwaveform (PES) obtained when the head reads the burst pattern intodigital data and effects the processing operation for deriving thepositional error by using the digital data. As technology advancesprovide smaller disk drives, and increased track densities, theplacement of servo-patterns becomes crucial for successful hard drivedesigns.

Servo-patterns are conventionally written by dedicated, externalservowriting equipment, and typically involve the use of large graniteblocks to support the disk drive and quiet outside vibration effects. Anauxiliary clock head is inserted onto the surface of the recording diskand is used to write a reference timing pattern. An external head/armpositioner with a displacement measuring device for positional feedbackis used to precisely determine transducer location and is the basis forburst placement and spacing of bursts in successive tracks.

FIG. 1A illustrates a conventional “quad” burst servo pattern, see belowwith reference to U.S. Pat. No. 5,381,281, which includes: (1) an ABservo burst set 20 and 30; and (2) a CD servo burst set 40 and 50. Thequad burst servo pattern is written radially within a servo sector andis shown horizontally in FIG. 1A for illustration. The differencesbetween adjacent edges of the A servo burst 20 and the B servo burst 30define the track center lines and the difference between track centerlines defines the track pitch (TP) as shown by 95. Five tracks areshown, i.e., n to n+4. As the read head moves over this servo pattern,position error signals are generated. In the ideal case, position errorsignals 60 and 70 are generated by this quad burst servo pattern.Position error signals (or “sensitivity signals”) are used to determinethe position of the head. For instance, position error signal 60represents the signals from pattern A minus the signals from pattern B(A−B) and is maximum when the head is in the middle of a burst A and isminimum when the head is in the middle of a burst B. Position errorsignal 70 represents the signals from pattern C minus the signals frompattern D (C−D) and is maximum when the head is in the middle of a burstC and is minimum when the head is in the middle of a burst D.

In the typical case, gaps between servo bursts and small read heads tendto “round” the tops and bottoms of the ideal position error signals 60and 70. Therefore, to remain within the linear portions of thesesignals, the head position control circuit follows a signal 80 that, ineffect, switches back and forth between the two signals 60 and 70. Theswitching occurs when the two signals 60 and 70 have the same magnitude,e.g., at points 90 a-90 g, thereby avoiding the nonlinear regionslocated within the tops and bottoms of signals 60 and 70 in thenon-ideal case. The first switch of a track occurs at TP/4 and the nextoccurs at TP/2.

FIG. 1B illustrates a case where the servo bursts are written using aseamless and untrimmed technique, e.g., the positions of burst A and Celements are independent of the positions of burst B and D elements.Because of the independent positioning, seamless writing techniquesoffer less error than seamed techniques. However, depending on theallowed tolerances, gaps between servo bursts and very small read headscan be expected. As a result of these gaps and due to small read heads,flat or dead zones can appear in the actual position errors signals 60′and 70′. In some cases, the flat or dead zones appear in both signals60′ and 70′ at the same head position, e.g., 100 a, 100 b and 100 c. Atthese positions, the head positioning control circuit is not able to useeither of the position error signals to determine the head locationbecause neither signal changes with head position. This can lead to afatal result for a modern disk drive.

According to U.S. Pat. No. 6,049,442, a servo pattern for use on a datastorage surface includes at least one track to minimize position errorduring positioning of a transducer over the data storage surface. Theservo pattern includes a plurality of servo burst fields of constantamplitude for defining a centerline of the track and for determining theposition of the transducer. According to this technique, at least oneservo burst field comprises N segments, where N is equal to or greaterthan 2, and each of the N segments is written with constant amplitude.The amplitude of the at least one servo burst field is then determinedas a function of the amplitudes of the N segments.

According to U.S. Pat. No. 5,381,281, a quadrature based embedded servocontrol system is described to realize a high track density,high-performance hard disk drive system. Each data sector includes agray code field spanning the entire width of the data track and aquad-servo burst pattern having first, second, third, and fourth servoburst fields distributed along the length of a portion of the datasector. The center point of the first, second, third, and fourth servobursts are sequentially offset from the adjacent burst by a radialdistance equivalent to one-half of the data track width. The quad-servoburst pattern is used with a track-following algorithm based on thequadrature value of (A+B)−(C+D) to obtain a substantially increasedservo lock range. A second gray code field extending substantially thewidth of the data track and second quad-servo burst patternsubstantially identical to the first is provided near a mid-point in thedata portion of the data sector to increase the servoing informationsample rate and accuracy, thereby permitting increased data trackdensities to be utilized.

According to U.S. Pat. No. 5,946,157, a rotating magnetic storage diskdrive is described having a method of seamlessly recordingcircumferentially overlapping servo bursts on a magnetic disk withsuccessive passes of a write head that is guided by a servo track writerwherein the servo bursts are contained in at least two servo burstgroups that each have at least one circumferential burst position whichmay contain a servo burst. The method includes turning a write currenton while passing the write head over a current ramp region that does notcontain servo data and is located in front of an “active” servo burstgroup that will be modified on this pass, modifying a servo burst in atleast one circumferential position of the active servo burst group withthe write current on. The method includes turning the write current offwhile passing the write head over a current ramp region that does notcontain servo data and is located in front of a “passive” servo burstgroup that will not be modified on this pass and then skipping over atleast one circumferential burst position of the passive servo burstgroup with the write current off.

However, what is needed is a servo mechanism that eliminates theproblems associated with non-linearities in the position error signalsbut also allows seamless and untrimmed servo burst writing techniqueswhich are better than above described art.

SUMMARY OF THE INVENTION

Accordingly, embodiments of the present invention provide a servomechanism that eliminates the problems associated with non-linearitiesin the position error signals but also allow seamless and untrimmedservo burst writing techniques. Seamless servo writing techniques areadvantageous as a result of position error cancellation in the servobursts. Embodiments of the present invention provide a seamless servoburst pattern that provides a linear position error signal used toimprove position sensitivity. The present invention provides theseadvantages and others not specifically mentioned above but described inthe sections to follow.

A disk storage system is described herein having servo data storedthereon for accurate head position control over a magnetic or otherstorage medium. The servo data is written using seamless and untrimmedwriting techniques and consists of a primary servo burst set (AB burstset) in which the difference between them defines the desired tractpitch for the surface. Multiple secondary servo burst sets (e.g., CDburst set and EF burst set) are written at the desired track pitch butradially offset from the primary burst set. In one embodiment, twosecondary servo burst sets are used, but alternatively, any number ofsecondary servo burst sets (two or greater) could be used.

In the exemplary embodiment having two secondary servo burst sets, theCD servo burst set is offset along its radial lines by ⅓ of the trackpitch from the primary servo burst set. Also in this embodiment, the EFservo burst set is offset along its radial lines by ⅔ of the track pitchfrom the primary servo burst set. There are six bursts per set. Theservo data is written in multiple servo sectors around the surface ofthe medium. From the servo data, three PES (position error signals)signals are generated, A−B, C−D and E−F, for providing linear positionsensitivity information to the head position control logic. Theresulting PES is more linear for smaller head sizes. The head positioncontrol logic follows the most linear path of these multiple PESsignals. In the embodiment using two secondary servo burst sets, the PESsignal switches at TP/6 (from the zero position) and at each time thetwo PES signals are equal in magnitude.

Embodiments of the present invention also include the use of more thantwo multiple secondary servo burst sets. For instance, if threesecondary servo burst sets were employed, then the first, second andthird secondary servo burst sets would be offset from the primary by ¼,½ and ¾ of a track pitch, respectively. The resulting PES signals wouldbe A−B, C−D, E−F and G−H and PES signal switching would take place atTP/8 (from the zero position), in this embodiment, and at each time thetwo PES signals are equal in magnitude.

Aspects of the present invention provide better linearity for the PESsignal while still maintaining the advantageous error cancellationcharacteristics of using seamless, untrimmed servo burst writingtechniques. The embodiments of the present invention are particularlyuseful in providing a linear PES signal even in applications that usesmall heads and have gaps between servo burst elements. In theseenvironments, the present invention provides improved positionsensitivity for the head position control logic.

More specifically, an embodiment of the present invention includes astorage medium having a disk surface that has concentric tracks and aplurality of servo sectors. Each sector has recorded therein servoinformation that is seamless and untrimmed. This servo information isused for track positioning of a head. The servo information contains aprimary burst set having a first servo data burst extending along afirst radial line and a second servo data burst extending along anadjacent second radial line and having bursts that are positioned inbetween bursts of the first servo data burst, the difference between theprimary burst set for defining positions of the concentric tracks. Theservo information also contains at least two secondary burst setsadjacent but radially offset from the primary burst set and eachradially offset from each other.

Embodiments also include the storage medium wherein the at least twosecondary burst sets comprise: a first secondary burst set radiallyoffset from the primary burst set and shifted along its radial line by ⅓track pitch from the primary burst set; and a second secondary burst setradially offset from the primary burst set and the first secondary burstset and shifted along its radial line by ⅔ track pitch from the primaryburst set.

Embodiments also include the storage medium wherein the first secondaryburst set comprises a third servo data burst and an adjacent butradially offset fourth servo data burst, the fourth servo data bursthaving bursts that are written in between bursts of the third servo databurst, the third and fourth servo data bursts for providing positionerror signals. Embodiments also include the storage medium wherein thesecond secondary burst set comprises a fifth servo data burst and anadjacent but radially offset sixth servo data burst, the sixth servodata burst having bursts that are written in between bursts of the fifthservo data burst, the fifth and sixth servo data bursts for providingposition error signals.

Embodiments include the storage medium wherein the primary burst set isseamless in that bursts of the first servo data burst are written inpositions that are independent of the positions of bursts of the secondservo data burst.

Embodiments include the storage medium wherein the at least twosecondary burst sets comprise: a first secondary burst set radiallyoffset from the primary burst set and shifted along its radial line by ¼track pitch from the primary burst set; a second secondary burst setradially offset from the primary burst set and the first secondary burstset and shifted along its radial line by 2/4 track pitch from theprimary burst set; and a third secondary burst set radially offset fromthe primary burst set and the first and second secondary burst sets andshifted along its radial line by ¾ track pitch from the primary burstset.

Embodiments also include a disk drive implemented in accordance with theabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates prior art seamless “quad” servo burst patterns andaccompanying graphs of ideal PES (position error signal) signals thatmay be generated therefrom.

FIG. 1B illustrates prior art seamless “quad” servo burst patterns andaccompanying graphs of PES signals that are generated therefrom and haveunwanted dead/flat spots therein.

FIG. 2 is a logical block diagram of relevant components of a hard diskdrive in accordance with embodiments of the present invention.

FIG. 3 is a diagram of a surface of a storage medium used in accordancewith an embodiment of the present invention illustrating data sectorsand servo sectors.

FIG. 4 illustrates a servo burst pattern in accordance with anembodiment of the present invention having a primary servo burst set(AB) and two secondary servo burst sets (CD and EF) radially offsettherefrom.

FIG. 5 illustrates a seamless writing technique used for recording theservo burst pattern of FIG. 4 in accordance with an embodiment of thepresent invention.

FIG. 6 illustrates the servo burst pattern in accordance with anembodiment of the present invention as shown in FIG. 4 and alsoillustrates the accompanying PES signals that switch at TP/6 (from thezero position) and each time the PES signals are equal in magnitude.

FIG. 7 illustrates a servo burst pattern in accordance with anotherembodiment of the present invention having a primary servo burst set(AB) and three secondary servo burst sets (CD, EF and GH) radiallyoffset therefrom.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present invention, aprimary servo burst set with multiple radially offset secondary servoburst sets for providing improved linear PES signals, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will be recognized by one skilled inthe art that the present invention may be practiced without thesespecific details or with equivalents thereof. In other instances, wellknown methods, procedures, components, and circuits have not beendescribed in detail as not to unnecessarily obscure aspects of thepresent invention.

FIG. 2 illustrates an exemplary hard disk drive HDD design 110 that maybe used in accordance with one embodiment of the present invention. Itis appreciated that aspects of the present invention may be practiced onmany different disk drive designs and that design 110 is only exemplary.HDD 110 includes reproduction/recording heads 106 each of which includesa read head 112 and a write head 108 and they may be mounted on aslider. The heads 106 are disposed to face the opposite surfaces of astorage medium, e.g., disk 104 and are held by an actuator 114. Theactuator 114 is a portion of a head driving mechanism which is driven torotate by a voice coil motor (VCM) 116 so as to move the head 106 in theradial direction to position the head. The VCM 116 is supplied with adriving current from a motor driver 122 which is controlled by aprocessor 124.

The motor driver 122 may drive both of the VCM 116 and a spindle motor102. The spindle motor 102 is a disk rotating motor for rotating thedisk 104 at high speed. One disk or a plurality of disks (in thisexample, one disk is used for convenience) are mounted on the spindlemotor 102 and the disk 104 has a large number of concentric tracksformed thereon as will be described later (FIG. 3).

Generally, a read/write circuit 120 of FIG. 2 is used for effecting thesignal processing operations necessary for the data read/writeoperation. A read signal read out by the read head 112 is amplified by ahead amplifier 118 and then supplied to the read/write circuit 120. Theread/write circuit 120 subjects the read signal to various signalprocessing operations such as an AGC (automatic gain control) amplifyingprocess, a filtering process, a digital conversion process, and adecoding process. The read/write circuit 120 converts the read signalinto reproduction data of NRZ code (write data) and transfers thereproduction data to a disk controller (HDC) 130. At the time of datawriting operation, the read/write circuit 120 effects the encodingprocess for modulating the NRZ data which is write data transferred fromthe HDC 130 into RLL (run-length coding) data, for example, and outputsthe modulated data to the write head 108 via the head amplifier circuit118.

Further, the read/write circuit 120 of FIG. 2 effects the process forreproducing servo information necessary for the head positioning controlin addition to the normal signal process for user data. As describedbefore, the servo information is constructed by cylinder data (trackaddress code) used for the seeking operation and servo burst datarelating to this invention. The cylinder data is recorded by use of Graycode. The read/write circuit 120 subjects a read signal (servo readsignal) corresponding to cylinder data read out by the read head 112 tothe binary-coding process and outputs the binary-coded signal to a servocircuit 126. The servo circuit 126 decodes the binary-coded signal intodata of cylinder address (track address) and outputs the decoded data tothe processor 124. The processor 124 recognizes the track in which thehead 112 is now set according to the cylinder data at the time ofseeking operation.

The read/write circuit 120 outputs a read signal corresponding to servoburst data read out by the read head 112 to the servo circuit 126 whileit is maintained in the analog signal state. As will be described later,the servo burst data may be an analog signal waveform indicating anamplitude value corresponding to the position of each burst pattern. Theservo circuit 126 samples values corresponding to respective burstpatterns and outputs position signals corresponding to the respectivevalues to the A/D converter of the processor 124.

The processor 124 is the main control device of the HDD 110 for headpositioning control may be a micro-controller with the A/D converter anda D/A converter used for outputting a control signal. The processor 124accesses a memory 128 to effect the head positioning control. In thememory 128, a control program (firmware) is stored for determining theoperation of the processor 124.

The processor 124 effects the seeking operation for moving a head 106 toa target track based on cylinder data decoded by the servo circuit 126.Further, the processor 124 receives the servo burst data as positiondata affects the operation for detecting the positional error. Based onthe results of the above operations, the processor 124 outputs a controlsignal for positioning the read head 112 to the desired target track andposition. That is, the processor 124 drives the VCM 116 via the motordriver 122 according to the control signal and controls the operation ofthe actuator 114 to position the head 106 to the target position.

The HDC 130 is a controller which is connected to the processor 124 andconstructs an interface with a host system to control the transfer ofread/write data (user data). The HDC 130 exchanges a read/write commandand read/write data with respect to the host system and controls abuffer memory (buffer RAM) 132 to buffer read/write data.

As shown in FIG. 3, a number of concentric tracks 160 are formed on bothsurfaces of the disk 104. In one embodiment, the tracks 160 are dividedinto a plurality of zones Z0 to Z2. In each of the tracks 160, aplurality of servo sectors 150 are arranged at regular intervals alongrespective radial lines. In the servo sector 150, an AGC (Automatic GainControl) area and a sector data area are provided in addition to thecylinder data area and a servo burst data area. The AGC area is an areain which data of constant frequency for stabilizing the amplitude of asignal is recorded. The sector data area is an area in which dataindicating a servo sector number is recorded. Further, in each of thetracks 160, a preset number of data sectors 170 are arranged between theservo sectors 150. The data sector 170 is an area for recording userdata. In one embodiment, the number of data sectors may be different foreach zone.

FIG. 4 illustrates servo information in accordance with one embodimentof the present invention. This servo information 425 is written withineach servo sector 150 (FIG. 3) and is shown in a horizontal orientationfor discussion. Each burst (A through F) extends along a radial linefrom the center of the disk toward an outer edge. In this embodiment,there are six bursts (A through F) per track that are used for headpositioning. Each burst set or pair, (A/B), (C/D) and (E/F), generatesits own position error signal which are used for head positioning. Basedon timing signal information placed on the disk, the head is made awareof when it is positioned over the radial line associated with one of thebursts.

More specifically, the servo information 425 of FIG. 4 consists of aprimary servo burst set which includes a first servo data burst 210(“A”) and an adjacent and radially offset second servo data burst 215(“B”). The difference between adjacent edges of the “A” and “B” burstsdefine the individual track locations 250. Differences in the tracklocations (e.g., track center lines) define the track pitch 240 for thedisk. As shown, there are some exemplary track centers (n) to (n+5) fordiscussion. The individual bursts of the first and second servo databursts 210 and 215 are positioned in between one another. That is, thesecond servo data burst 215 (“B”) is shifted along its radial line by afull track pitch (TP) with respect to the first servo data burst 210(“A”) and small gaps may exist between the individual bursts of theseburst sets.

In accordance with the present invention, the servo information 425 ofFIG. 4 also contains multiple secondary servo burst sets which areadjacent to the primary servo burst set, but offset radially and shiftedalong their radial lines. For instance, a first secondary servo burstset includes a third servo data burst 220 (“C”) and an adjacent andradially offset fourth servo data burst 225 (“D”). The individual burstsof the third and fourth servo data bursts 220 and 225 are positioned inbetween one another. Importantly, the first secondary servo burst set(C/D) is offset from the primary servo burst set (A/B) by ⅓ of a trackpitch (TP/3) along their radial lines. Also, a second secondary servoburst set includes a fifth servo data burst 230 (“E”) and an adjacentand radially offset sixth servo data burst 235 (“F”). The individualbursts of the fifth and sixth servo data bursts 230 and 235 arepositioned in between one another. Importantly, the second secondaryservo burst set (E/F) is offset from the primary servo burst set (A/B)by ⅔ of a track pitch (⅔ TP) along their radial lines.

In the example of FIG. 4, two secondary data burst sets are shown, butit is appreciated that the present invention may operate with any numberof additional secondary data burst sets. For instance, FIG. 7illustrates an example having three secondary data burst sets, e.g.,(C/D), (E/F) and (G/H), and a primary data burst set. As described inmore detail below, the multiple secondary data burst sets of the presentinvention provide increased position error signal information that isused by the servo positioning control circuit 126 (FIG. 2) and theprocessor 128 (FIG. 2). By increasing the position error signalinformation, a more linear PES signal can be established and thereforethe head 106 (FIG. 2) can be accurately positioned even if gaps betweenthe individual servo bursts exist and small read heads are used.

It is appreciated that the ordering of the bursts, e.g., A through F isonly exemplary and is presented only for discussion. It is appreciatedthat the burst order is arbitrary and the ordering of the bursts may berearranged according to any order.

FIG. 5 illustrates the embodiment 425 in the vertical orientation havinga primary servo burst set (A/B) and two secondary servo burst sets (C/D)and (E/F). In one embodiment of the present invention, the bursts of theservo information 425 are written using a seamless writing technique andare generally untrimmed meaning the edges of the bursts are fringedthereby creating small gaps between bursts of the set (e.g., between Aand B). The track centers 250 are also shown which are defined by thedifference between the adjacent edges of the “A” and “B” bursts.

Using a seamless servo writing technique, the bursts (“A”) of servo databurst 210 are written independently of the bursts (“B”) of servo databurst 215. In other words, in write cycle i, burst A is written and thenin cycle i+1, burst B is written. Therefore, errors in the positions ofthe A bursts are totally independent of the errors in the positions ofthe B bursts. In other words, the random error in the position of the Aburst are independent from the random errors in the position of the Bburst. In this way, using a seamless technique, the PES signal derivedby (A−B) tends to statistically cancel out the uncorrelated errors, 260a and 260 b, associated with the positions of the A bursts and the Bbursts. That is, 260 b represents the amount of error, from the idealposition, associated with the position of individual burst B and 260 arepresents the amount of error, from the ideal position, associated withthe position of individual burst A.

Further, by taking the PES signal derived by (A−B), random errors in thetrack pitch are insensitive to the error component of the B burst whichis correlated to the errors on the two edges of the B burst. This isbecause burst B is written in a separate write cycle from burst Aaccording to seamless servo writing techniques whereby each burst iswritten separately and independently from the others. This is incontradiction to seamed servo writing techniques where the tail end ofone individual burst is erased during the write cycle of another,adjacent, individual burst thereby tying the positions of the twoindividual bursts together.

FIG. 6 illustrates the servo information 425 (oriented horizontally) inaccordance with an embodiment of the present invention along with agraph of the three position error signals 310-320 used during headpositioning. As discussed above, the primary servo burst set (A/B)generates an A−B position error signal 310 that is maximum when the readhead is in the middle of burst A and is minimum when the read head is inthe middle of burst B. The first secondary servo burst set (C/D)generates a C−D position error signal 315 that is maximum when the readhead is in the middle of burst C and is minimum when the read head is inthe middle of burst D. Lastly, the second secondary servo burst set(E/F) generates an E−F position error signal 320 that is maximum whenthe read head is in the middle of burst E and is minimum when the readhead is in the middle of burst F. Although shown in the ideal case, itis appreciated that the top and bottom tips of the position errorsignals 310-320 are actually somewhat rounded and non-linear due to gapsbetween individual bursts and the small size of many read heads.

Although receiving all three PES signals 310-320, the servo positioninglogic of the disk drive utilizes the position error signal 330 that isshown in the dotted line, for positioning the head 106 (FIG. 2).Essentially, PES 330 follows the linear portions of curves 310-320 andnever ventures into the top or bottom tips which contain the non-linearregions. Specifically, PES 330 switches from one curve to another whenthe magnitudes of two of the curves are equal. As shown, with respect totrack n, the PES 330 starts on curve 310 and switches to curve 315 afterTP/6 (from the track start or zero position) and then switches to curve320 after TP/3 and then switches to curve 310 after TP/3, etc. In anygiven track, the PES 330 switches at TP/6 (from the track start or zeroposition) and then switches after each TP/3. By switching in theseamounts, the present invention guarantees that the non-linear regions ofthe position error signals can be avoided while still using a seamless,untrimmed servo writing technique and while still using small readheads.

FIG. 7 illustrates another embodiment of the present invention includingservo information 450 that utilizes three secondary servo burst sets(C/D), (E/F) and (G/H) adjacent to the primary servo burst set (A/B).Each burst extends along a radial line from the center of the disktoward an outer edge. In this embodiment, there are eight bursts pertrack that are used for head positioning. The primary servo burst set(A/B) includes the “A” burst pattern 210 and the “B” burst pattern 215.The first of the secondary burst sets includes the “C” burst pattern 220and the “D” burst pattern 225. The “C” burst pattern 220 is offset byTP/4 along its radial line from the primary servo burst set (A/B). Thesecond of the secondary burst sets includes the “E” burst pattern 230and the “F” burst pattern 235. The “E” burst pattern 230 is offset byTP/2 along its radial line from the primary servo burst set (A/B).Lastly, the third of the secondary burst sets includes the “G” burstpattern 410 and the “H” burst pattern 415. The “G” burst pattern 410 isoffset by ¾ TP along its radial line from the primary servo burst set(A/B).

With respect to embodiment 450, four position error signals aregenerated, (A−B), (C−D), (E−F) and (G−H). The servo position controllogic switches between these error signals when their magnitudes areequal. Within a given track, the first switch occurs at TP/8 from thezero position. Each other switch within the track is then performed atTP/4.

It is appreciated that more than 3 secondary burst sets can be used inaccordance with the scope of the present invention. However, as moresecondary burst sets are used, the efficiency of the servo trackingsystem is reduced. Generalizing the present invention, if n secondaryburst sets are used, then each individual burst set is offset from theprimary burst set by a multiple of 1/n. The resulting PES signal thenswitches from the zero position at TP/(2n).

The preferred embodiment of the present invention, a primary servo burstset with multiple radially offset secondary servo burst sets forproviding improved linear PES signals, is thus described. While thepresent invention has been described in particular embodiments, itshould be appreciated that the present invention should not be construedas limited by such embodiments, but rather construed according to thebelow claims.

What is claimed is:
 1. A storage medium comprising: a disk surfacecomprising concentric tracks and a plurality of servo sectors eachhaving recorded therein servo information that is seamless anduntrimmed, said servo information for track positioning of a head; andwherein said servo information comprises: a primary burst set comprisinga first servo data burst extending along a first radial line and asecond servo data burst extending along an adjacent second radial lineand comprising bursts that are positioned in between bursts of saidfirst servo data burst, said primary burst set for defining positions ofsaid concentric tracks; and at least two secondary burst sets adjacentbut radially offset from said primary burst set and each radially offsetfrom each other.
 2. A storage medium as described in claim 1 whereinsaid at least two secondary burst sets comprise: a first secondary burstset radially offset from said primary burst set and shifted along itsradial line by ⅓ track pitch from said primary burst set; and a secondsecondary burst set radially offset from said primary burst set and saidfirst secondary burst set and shifted along its radial line by ⅔ trackpitch from said primary burst set.
 3. A storage medium as described inclaim 2 wherein said first secondary burst set comprises a third servodata burst and an adjacent but radially offset fourth servo data burst,said fourth servo data burst comprising bursts that are written inbetween bursts of said third servo data burst, said third and fourthservo data bursts for providing position error signals.
 4. A storagemedium as described in claim 3 wherein said second secondary burst setcomprises a fifth servo data burst and an adjacent but radially offsetsixth servo data burst, said sixth servo data burst comprising burststhat are written in between bursts of said fifth servo data burst, saidfifth and sixth servo data bursts for providing position error signals.5. A storage medium as described in claim 1 wherein said first andsecond servo data bursts of said primary servo burst set also provideposition error signals.
 6. A storage medium as described in claim 1wherein said primary burst set is seamless in that bursts of said firstservo data burst are written in positions that are independent of thepositions of bursts of said second servo data burst.
 7. A storage mediumas described in claim 1 wherein said at least two secondary burst setscomprise: a first secondary burst set radially offset from said primaryburst set and shifted along its radial line by ¼ track pitch from saidprimary burst set; a second secondary burst set radially offset fromsaid primary burst set and said first secondary burst set and shiftedalong its radial line by 2/4 track pitch from said primary burst set;and a third secondary burst set radially offset from said primary burstset and said first and second secondary burst sets and shifted along itsradial line by ¾ track pitch from said primary burst set.
 8. A diskdrive comprising: a storage medium comprising concentric tracks and aplurality of servo sectors each having recorded thereon servoinformation that is seamless and untrimmed; a head for reading andwriting data from said storage medium; and a head positioning mechanismfor positioning said head over said concentric tracks based on saidservo information; and wherein said servo information comprises: aprimary burst set comprising a first servo data burst extending along afirst radial line and a second servo data burst extending along anadjacent second radial line and comprising bursts that are positioned inbetween bursts of said first servo data burst, said primary burst setfor defining positions of said concentric tracks; and at least twosecondary burst sets adjacent but radially offset from said primaryburst set and each radially offset from each other.
 9. A disk drive asdescribed in claim 8 wherein said at least two secondary burst setscomprise: a first secondary burst set shifted along its radial line by ⅓track pitch from said primary burst set; and a second secondary burstset shifted along its radial line by ⅔ track pitch from said primaryburst set.
 10. A disk drive as described in claim 9 wherein said firstsecondary burst set comprises a third servo data burst and an adjacentbut radially offset fourth servo data burst, said fourth servo databurst comprising bursts that are written in between bursts of said thirdservo data burst, said third and fourth servo data bursts for providingposition error signals.
 11. A disk drive as described in claim 10wherein said second secondary burst set comprises a fifth servo databurst and an adjacent but radially offset sixth servo data burst, saidsixth servo data burst comprising bursts that are written in betweenbursts of said fifth servo data burst, said fifth and sixth servo databursts for providing position error signals.
 12. A disk drive asdescribed in claim 8 wherein said first and second servo data bursts ofsaid primary servo burst set also provide position error signals.
 13. Adisk drive as described in claim 8 wherein said primary burst set isseamless in that bursts of said first servo data burst are written inpositions that are independent of the positions of bursts of said secondservo data burst.
 14. A disk drive as described in claim 8 wherein saidat least two secondary burst sets comprise: a first secondary burst setshifted along its radial line by ¼ track pitch from said primary burstset; a second secondary burst set shifted along its radial line by 2/4track pitch from said primary burst set; and a third secondary burst setshifted along its radial line by ¾ track pitch from said primary burstset.